There are many applications in industry and science in which it is required to determine the density of particles suspended in a fluid (i.e., liquid or gaseous) medium, and the sizes or size distribution of such particles. Such measurements can be very important in manufacturing processes in a number of industries, including those related to pharmaceutical, plastics, chemical, food processing, mining, ceramic, and concrete aggregate technologies. Processes such as crystal growth, precipitation, polymerization, gravimetric separation, and grinding must be monitored and/or controlled with regard to the sizes of suspended particles to control the quality of the final product.
While particle size analyzers have traditionally been used as a final quality control tool, recent market studies have shown that customers are increasingly looking at on-line particle size control to make process adjustments so as to prevent off-grade materials from being produced. Specific applications include: (a) the food processing industry, e.g., in monitoring and controlling the dry grinding of flours, coffee, sugar, and other ingredients, any of which can dramatically change the taste properties of food products in which they are incorporated; (b) the coal-fired power industry, where the size of coal dust injected into a power boiler combustion chamber determines the efficiency rating of that boiler and the extent to which noxious by-products like CO and NO.sub.2 are produced. In all of these cases, close monitoring of the process materials, in real time, enhances production efficiency, which quickly pays for the capital cost of the particulate size monitoring and control system. To produce such on-line particle size monitoring and control systems, one has to have a reliable and continuously operating measuring system that incorporates a reliable sampling system (to continuously extract material from a production flow line), a sample reduction system to reduce a larger sample to a smaller measurable sample (riffler), and a continuous, automatically operating particle size measuring device.
In the past, a number of different technologies have been used for the analysis of particle size and particle size distribution. The most widely used technique involves screen or sieve analysis. In this technique, a representative sample of a particulate product is poured into a stack of sieves or screens, each screen being attached to the bottom of a tray. The stack of screens is then shaken or vibrated for a predetermined length of time, causing the particles to fall through the screens until the size of the mesh in a particular screen is less than the size of a particle, stopping the particle on that screen. The contents of each tray is then weighed, and the size fraction in each tray is expressed as a percentage of the total weight of the specimen. The screens are arranged in order sequentially by their respective sizes so that the largest particles will be retained on the first screen, the next largest on the second screen, and the smallest particles fall through all of the screens and are collected in a tray at the bottom of the stack of screens. Each of the screens is characterized by specific mesh size. In a standard range, there are 30 different mesh sizes, from mesh size #4, equivalent to a particle diameter of 4.76 mm (0.187 inches) to mesh size #325, equivalent to a particle diameter of 0.044 mm (0.0017 inches). It should be noted that the #325 mesh size is typically the smallest mesh size commonly used; as a consequence, screens are normally used to segregate particles ranging in size from 0.044 mm (0.0017 inches) up to several mm in size. While screens are relatively inexpensive, they have several significant limitations in sizing particulate matter, specifically:
(1) Screens are very labor intensive to use. The procedures for filling the screens, weighing each tray after each screening operation, and cleaning the trays all involve manual operations, which require operator time. PA1 (2) A sieving operation typically only involves up to a maximum of eight size trays, thereby limiting the resolution of the measurement. PA1 (3) The screens wear and have to be replaced periodically. PA1 (4) Screens used in a dry-sieving operation are often affected by electro-static effects. The shaking and vibrations during the sieving operation result in electro-static charge effects that cause the particles to adhere to each other, producing erroneous results. PA1 (1) Laser diffraction devices require the light to be transmitted through the sensing zone region from a light source at one side of the sensing zone toward detectors mounted at an opposite side. Therefore, the concentration of particles has to be sufficiently low so that the light is able to penetrate the assemblage of particles suspended in the measuring region. Laser diffraction devices require complex dilution and sample conditioning systems to ensure that the concentration of particles meets this requirement. PA1 (2) The amount of light scattered from larger particles due to Fraunhofer diffraction is, on a percentage basis, much less as compared to smaller particles; also, for larger particles, the scattering angle is very close to 0.degree., substantially aligned with the direction of propagation of the collimated beam. Consequently, the measuring range for large particles is typically limited to about 500.mu. (0.5 mm, 0.0197 inch). PA1 (3) Laser diffraction devices are complex devices, requiring a dedicated computer for data handling, and typically have a cost about 20 times that of simple sieve systems.
The second most widely used technique to characterize particle size and size distribution is an optical technique using the Fraunhofer diffraction effect. There are a number of companies making devices based on this technique, e.g., Malvern Ltd. (MASTERSIZER.TM.), Leeds & Northrup (MICROTRAC II.TM.), Cilas (GRANULOMETER.TM.), Coulter Co. (LS 100.TM.). These devices use a coherent laser beam to produce a collimated light beam that is transmitted across a sensing region. Particles that flow (or are blown) across this sensing region diffract light from the collimated beam. The diffracted light is scattered out of the collimated light beam and is collected by a set of lenses, which in turn focus this diffracted light onto a plurality of detectors. These detectors are typically arranged in a concentric ring configuration in order to capture all of the light scattered at a given angle. In accordance with standard Fraunhofer diffraction theory, smaller particles scatter light through a larger scattering angle than larger particles. This type of device is best suited to measure particle sizes in the range of 0.5.mu. (0.0005 mm) to 500.mu. (0.5 mm, 0.0197 inch).
Laser diffraction devices have several limitations, which make them more costly and difficult to use compared to sieving devices (particularly for particle size measurements above 500.mu.), including the following:
Both the sieve and the laser diffraction techniques have significant drawbacks, as described above. The former technique can not readily be automated and the latter technique does not measure the size distribution over the full particulate size range (from 0.031-4.76 mm) covered by standard sieving technique.
A third measuring technique is also based on a light transmission geometry to measure the amount of light that is obscured by individual particles as they pass through an optical sensing region. In such devices (commercially available from several companies, e.g., Climet and Hyac Royco), the size of each particle is proportional to the amount of light obscured by the cross-sectional area of that particle, measured as a percentage fraction of the total cross-sectional area of the sensing region. This relationship is based on area, both the area of a particle (i.e., the product of its cross-sectional length and width) and the area of measuring region. If the size of the particle is 1/100 of the width of the sensing region measured transverse to the light path, then the degree of light obscuration produced by the total surface area of the particle as it passes through the sensing region is [1/100].sup.2, or 1 part in 10,000. This technique typically covers measurement of a particle size ranging from 0.5.mu. through 100.mu. and is limited to single particle measurements. Because the area of the sensing region is relatively large compared to the area of particles at the low end of the size range, the probability of more than one particle at a time passing through the sensing region is very high. As a practical result, this technique can only be used at very low particle concentrations, e.g., for use as an impurity monitor.
A modification of the light obscuration principle of measurement is disclosed in U.S. Pat. No. 4,842,406 (VonBargen). In this patent, a collimated sheet of light is directed through a sensing region to monitor particles in three size ranges, including 0.5.mu. through 10.mu., 10.mu. through 50.mu., and 50.mu. through 300.mu.. The particles in the smallest size range are detected using a first detector that responds to forward scattered light; particles in the mid-size range are detected by a second detector that is disposed transverse to the collimated sheet of light, as a function of the amplitude of pulses produced by the direct beam as a particle intersects the collimated sheet of light. The larger particles in the third range are detected by measuring the duration of pulses in the direct path using the first detector. The patent describes three different measuring techniques that applied to "measure particle sizes most precisely in the entire range from 0.5.mu. to 300.mu.," in other words, to widen the dynamic range of the measurement as much as possible. Still, the described measuring technique is limited in that it (a) only measures single particle events, or relatively low particle concentration (e.g., at the extremes of the size range it can only measure at [(0.5/300).sup.2 .times.100=]0.00027% concentration because of the area of the measuring region relative to the area of the particle, as discussed above), (2) requires constant material flow velocity through the measuring region, (3) does not cover the higher size ranges above 300.mu., and (4) is not linear over the entire measuring range. The present invention overcomes these limitations by recognizing that as long as the particles passing through the collimated sheet of light have a larger size compared to the thickness of the sheet of light, the size of an individual particle is proportional to the amount of light it obscures as it passes through the sheet of light. Expressed as a percentage fraction of the total line-width of the sheet of light, particle size is simply the size of that portion of the particle that is illuminated by the sheet of light. This relationship is then directly a function of particle size, not, as in the other related light obscuration techniques discussed above, a function of the square of particle size. If the size of the particle is 1/100 of the size of the sensing region, then the amount of light obscuration is indeed 1/100 or 1 part in 100. The advantage of this type of obscuration technology is two-fold: (1) particles pass through a thin sheet of light faster than through a 3-dimensional volume of light, i.e., the coincidence rate is lower, and both higher counting rates and higher material concentrations can be obtained; and (2) a wider dynamic range of particle sizes can be measured.
U.S. Pat. No. 4,842,406 touches on the effect of particles larger than the width of a sheet of light in discussing the detection of particles in the size range from 50 through 300.mu., but instead of using an amplitude measurement (indicating the percentage of obscuration caused by a large particle), the patent relies upon a measurement of pulse width or duration. This approach necessarily requires that the material flow speed be maintained very constant. Moreover, the reference does not teach how to avoid errors caused by more than one particle obscuring a portion of the sheet of light at a time. Accordingly, this technique is not usable in applications where higher material concentrations are encountered and where sieves are more commonly used, i.e., in applications where particle sizes range from 44.mu. up to several mm.
It is clear that to make full use of the light obscuration technique, it is important to produce a sheet of light, which in the measuring region, is very thin, very wide, and has high uniformity across its entire width, without the diffraction effects normally associated with such light beam configurations. The reasons are as follows: (a) the "thinness" of the light sheet determines the sensitivity to the smallest particle size, (b) the "width" of the sheet determines the maximum particle size that can be measured, and (c) the "uniformity" of the light intensity determines the accuracy of the measurement as a function of the position of a particle across the sheet of light. Diffraction effects are typically caused by using a slit to define the shape of the light beam and produce spurious multiple pulses as a particle falls through the diffracted sheet of light.
The present invention is intended to overcome the limitations and problems in the above-described types of light obscuration systems, including the limitations of the relatively complex, three size range approach disclosed in U.S. Pat. No. 4,842,406, and to provide a cost effective system to fill the void between the particle size and concentration capabilities of existing sieving and laser diffraction techniques. The present invention is further intended to provide a reliable and reproducible technique that can be used by unskilled operators in a standard testing lab environment. It is another objective of this invention to provide a reliable and continuous measuring system for use with a continuously flowing process stream, to provide a continuously updated reading of particle size, so that specific features of the particle size distribution can then be used for process control.